Siegen, Universiẗat, Diss., 2002.

Thesis (M.S.)--West Virginia University, 2006. / Title from document title page. Document formatted into pages; contains ix, 98 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 87-91).

Universiẗat, Diss., 2000--Frankfurt (Main).

Thesis (Ph. D.)--Lehigh University, 2000. / Includes vita. Includes bibliographical references (leaves 246-255).

Thesis (MScIng)--University of Stellenbosch, 2008. / Bibliography. Also available via the Internet.

Nucleation and propagation of cracks under fretting conditions has been a subject of study for many years. An extensive experimental investigation to study these cracks was undertaken by Royal Aerospace Establishment (RAE Farnborough). Of particular interest to RAE was an Aluminium alloy (L65) developed for aerospace applications. Many researchers have studied fretting damage and fatigue cracks. Some have examined damage development due to wear, whilst others have analysed cracks under linear elastic fracture mechanics (LEFM) domain. To date, no attempt has been made to develop an integrated numerical model which incorporates all aspects of fretting fatigue i.e. nucleation, initial (or early) crack growth, and long crack propagation. The development of such a model is the principal aim of this work. It is expected that the integrated approach will provide the basis for a standard fretting fatigue analysis of other materials, components, and structures using the finite element method (FEM).This study uses the earlier experimental results with RAE as the reference for comparison. The approach followed is to implement the various stages of fretting in a commercial finite element code, ABAQUS. Unlike previously used simple FE models, both specimen (Aluminium alloy) and the fretting pad (steel) are modelled to simulate the real contact conditions including slip. Various predictive models for crack nucleation (based on damage) and propagation (based on fracture mechanics) are developed, tested, and implemented in ABAQUS. Results clearly show that these models together provide a good estimation tool for predicting total life in complete contact fretting fatigue. It is envisaged that the integrated model will be easily utilised for other materials, components, and structures subjected to fretting fatigue conditions with minimum experimental testing required.

No description available.

University of Macau / Faculty of Science and Technology / Department of Civil and Environmental Engineering

Abstract This thesis deals with the analysis of the rheological structure and tectonic modelling of the Fennoscandian Shield. First, a short introduction to the geology and geophysics of the Fennoscandian Shield is presented followed by a description of rheological concepts. Second, the applied modelling procedures, together with the sources of error are explained. Last a brief summary of each original paper including conclusions is given. Understanding rheological conditions through the entire lithosphere and even deeper is the key for understanding the deformation of the earth's interior. Thus, investigating the rheological structure and possible consequences resulting from tectonic loading are required to some extent when interpreting geophysical data into tectonic models. In this thesis rheological structure is obtained by calculating rheological strength in different locations of the central Fennoscandian Shield. These locations are mainly situated along different deep seismic sounding (DSS) profiles as they provide necessary geophysical information required for model construction. Modelling begins by solving the thermal structure in the lithosphere, as rheological behaviour, mainly ductile flow is strongly controlled by temperature. Results from these calculations show that the rheological structure of the lithosphere depends on the thermal conditions resulting in significant areal variations. Generally, the central Fennoscandian Shield can be considered to be rheologically rather strong. Rheologically weak layers are however usually found in the lower crust. Correlation of the rheological structure with earthquake focal depth data shows that brittle fracture is the relevant mechanism in the earthquake generation and that non-occurrence of deep earthquakes implies low stress or high strength conditions deeper in the crust. Calculated rheological structure is furthermore used as a material parameter in the structural models which are solved next. These results suggest that it is highly unlikely that any considerable ductile deformation in the crust of the central Fennoscandian Shield exists and it seems that the present-day thermal and mechanical conditions in the investigated area do not favour such processes in significant amounts.

SHS vs. Gamma Nail Using FE analysis, the behaviour of the Gamma nail and the SHS was compared in an osteoporotic bone model for the fixation of three- and fourpart trochanteric fractures (31-A2 in the AO classification, types IV and V in Evans’ classification). The size of the medial fragment was varied based on clinical data and the case of a fractured greater trochanter was also considered. Results show that for Evans’ type V stabilized with a Gamma nail and for Evans’ types IV and V with the SHS, cancellous bone around the lag screw is susceptible to yielding, thus indicating a risk of cut-out. The volume of bone susceptible to yielding increases with an increase in size of the medial fragment. On the other hand, Evans’ type IV with a Gamma nail is not predicted to cut out. These findings suggest that future clinical trials investigating fixation of unstable proximal fractures should include the size of the medial fragment and the integrity of the greater trochanter as covariables and be powered to evaluate whether intramedullary devices are superior to SHSs for Evans’ type IV fractures and inferior/equivalent for type V. PFNA Because of the contradictory body of evidence related to the potential benefits of helical blades in trochanteric fracture fixation, the e↵ect of bone compaction resulting from the insertion of a PFNA was evaluated. A subject-specific computational model of a trochanteric fracture (31-A2 in the AO classification) was developed with lack of medial support and bone density was varied to account for variability in bone properties amongst hip fracture patients. Results show that for a bone density corresponding to 100% of the bone density of the cadaveric femur, there does not seem to be any advantage in using a PFNA with respect to the risk of blade cut-out. On the other hand, in a more osteoporotic femoral head characterized by a density corresponding to 75% of the initial bone density, local bone compaction around the helical blade provides additional bone purchase, thereby decreasing the risk of cut-out, as quantified by the volume of bone susceptible to yielding. These findings indicate benefits of using a PFNA over an intramedullary nail with a conventional lag screw and suggest that any clinical trial reporting surgical outcomes regarding the use of helical blades should include a measure of the femoral head bone density as a covariable. TAD and Screw Position Using a simple mathematical formulation, the relationship between the position of the lag screw tip (relevant to both intramedullary and extramedullary devices) and the concept of tip-apex distance (TAD) was derived. TAD is widely used in operating theaters as a surgical guideline in relation to the fixation of trochanteric fractures and in clinical studies as a predictor of lag screw cut-out. In order to visualize better this concept, the locus of points having the same TAD was plotted and TAD variations as the location of the lag screw tip was varied were described. Findings show that TAD should be adjusted for the size of the femoral head (a variable which varies a lot according to the sex of the patient) and that no correlation exists between TAD and bone morphometry indices obtained from μCT data (BV/TV and Tb.Th). Therefore, these results seem to suggest that TAD lacks mechanical justification and that predictors which are based on mechanical properties, such as bone density, should be investigated further. Cut-out and Screw Position The biomechanical performance of a CT scan-based three-part trochanteric fracture model (31-A2 in the AO classification) stabilized with a SHS was compared for nine di↵erent positions of the lag screw (3 x 3 arrangement, from anterior to posterior and from inferior to superior). Results show that the volume of bone susceptible to yielding in the head and neck region is the lowest for inferior positions and increases as the lag screw is moved superiorly. Overall, for this specific subject, the models less likely to lead to cut-out are the ones corresponding to inferior middle and inferior posterior positions of the lag screw. In this study, TAD was anti-correlated with the risk of cut-out, as quantified by the volume of bone susceptible to yielding, which suggests that a TAD > 25 mm cannot be considered to be an accurate predictor of lag screw cut-out. Further clinical studies investigating lag screw cut-out should attempt to find more reliable predictors of cut-out that should better reflect the biomechanics and subject-specificity of the femoral head. Plasticity Formulations Using an FE model of unstable trochanteric fracture stabilized with a SHS, the benefits of two plasticity-based formulations, Drucker-Prager and crushable foam, were evaluated and compared to the commonly used linear elastic model of trabecular bone in order to predict relative risk of lag screw cut-out for five distinct load cases. The crushable foam plasticity formulation leads to a much greater strain localization, in comparison to the other two models, with large plastic strains in a localized region. The plastic zone predicted with Drucker-Prager is relatively more di↵use. Linear elasticity associated with a minimum principal strain criterion provides the smallest volume of elements susceptible to yielding for all loading modes. The region likely to undergo plastic deformation, as predicted by the linear elastic model, is similar to that obtained from plasticity-based formulations, which indicates that this simple criterion provides an adequate estimate of the risk of cut-out.